Prior art engines have utilized elongated co-acting rotors of elliptical cross section to define combustion changers that expand and contract as they rotate in unison. Hollow rotors with hollow supporting shafts have been used, with intake ports formed thru the sides of part of the rotors, and exhaust ports formed thru others of the rotors. Other engines have circulated cooling liquid thru hollow rotors, still others have circulated cooling air thru hollow rotors.